This invention relates generally to detecting the passing of a plug released into a well. The plug typically is a cementing plug pumped into an oil or gas well with cement and other fluids containing various materials, and the invention preferably is able to distinguish when the cementing plug rather than other material passes a particular location.
To support the upper part of an oil or gas well, a string of tubular segments called casing is typically cemented into place at the top of the well bore. The casing is lowered into the bore and then cement is pumped down the center of the casing and back up the annulus between the outside of the casing and the surface of the well bore. To move a desired amount of cement up into the annulus without leaving the interior of the casing filled with cement, a cementing plug is released from a closed plug container into the casing behind the cement once the volume of cement necessary to fill the annulus has been pumped into the well; and then a displacing fluid, which will not solidify inside the well, is pumped in on top of the cementing plug to push the cement into the annulus. The cementing plug separates the different fluids. Additional plugs and fluids can be used in series.
Because the fluids and plugs are moved into the well through a closed, pressurized system, an operator typically cannot directly see a plug release from the plug container and enter the fluid stream at the desired time. To know that a plug has actually entered the fluid stream is important because if the plug is not properly placed but the following fluid is pumped anyway, the following fluid may be pumped directly against the leading fluid thereby allowing possibly deleterious mixing of the different fluids. Additionally, if the plug is not properly placed, it will not be at the appropriate place to hit bottom and cause a detectable pressure increase for alerting an operator that displacement is complete; therefore, without such pressure increase signal being generated, over or under displacement of the fluid can result. Accordingly, there is the need for an apparatus and method for detecting that the plug has been properly released into the fluid stream.
Although the foregoing is known in the oil and gas industry and attempts have been made to provide plug detector apparatus and methods, there is the need for an improved apparatus and method. Such an apparatus and method should be non-invasive. That is, they should not mechanically cross the interior surface defining the closed flow path through which the plug and fluids move, thereby avoiding sealing problems. Such an apparatus and method should preferably distinguish the plug from other objects or materials in the flow. They also should preferably sense proper direction of plug movement. Furthermore, such apparatus and method preferably should use relatively low energy.